Abrams' heavy armor is up front, however, insurgents sneak up from behind, fire from rooftops above and set off mines below.

WASHINGTON — The U.S. military's Abrams tank, designed during the Cold War to withstand the fiercest blows from the best Soviet tanks, is getting knocked out at surprising rates by the low-tech bombs and rocket-propelled grenades of Iraqi insurgents.

In the all-out battles of the 1991 Gulf War, only 18 Abrams tanks were lost and no soldiers in them killed. But since the March 2003 invasion of Iraq, with tanks in daily combat against the unexpectedly fierce insurgency, the Army says 80 of the 69-ton behemoths have been damaged so badly they had to be shipped back to the United States.

At least five soldiers have been killed inside the tanks when they hit roadside bombs, according to figures from the Army's Armor Center at Fort Knox, Ky. At least 10 more have died while riding partially exposed from open hatches. (Related story: Tanks adapted for urban fights they once avoided)

The casualties are the lowest in any Army vehicles, despite how often the Abrams is targeted — about 70% of the more than 1,100 tanks used in Iraq have been struck by enemy fire, mostly with minor damage.

..........................Because it was designed to fight other tanks, the Abrams' heavy armor is up front. In Iraq's cities, however, insurgents sneak up from behind, fire from rooftops above and set off mines below.

A favorite tactic: detonating a roadside bomb in hopes of blowing the tread off the tank. The insurgents follow with rocket-propelled grenades, mortars and gunfire aimed at the less-armored areas, especially the vulnerable rear engine compartment...................................

"Be wary of eliminating or reducing ... heavy armor" as the Army modernizes, the officers warn, arguing it is crucial against insurgents' "crude but effective weapons."

The Army says most of the "lost" tank hulls can be rebuilt and returned to battle someday. Meanwhile, the Army is upgrading the Abrams, including better protection for the tank's engine compartment.

The Army has deployed a new troop transport vehicle in Iraq with many defects, putting troops there at unexpected risk from rocket-propelled grenades and raising questions about the vehicle's development and $11 billion cost, according to a detailed critique in a classified Army study obtained by The Washington Post.

The vehicle is known as the Stryker, and 311 of the lightly armored, wheeled vehicles have been ferrying U.S. soldiers around northern Iraq since October 2003. The Army has been ebullient about the vehicle's success there, with Gen. Peter J. Schoomaker, the Army chief of staff, telling the House Armed Services Committee last month that "we're absolutely enthusiastic about what the Stryker has done."

But the Army's Dec. 21 report, drawn from confidential interviews with operators of the vehicle in Iraq in the last quarter of 2004, lists a catalog of complaints about the vehicle, including design flaws, inoperable gear and maintenance problems that are "getting worse not better." Although many soldiers in the field say they like the vehicle, the Army document, titled "Initial Impressions Report -- Operations in Mosul, Iraq," makes clear that the vehicle's military performance has fallen short.

The internal criticism of the vehicle appears likely to fuel new controversy over the Pentagon's decision in 2003 to deploy the Stryker brigade in Iraq just a few months after the end of major combat operations, before the vehicle had been rigorously tested for use across a full spectrum of combat.

The report states, for example, that an armoring shield installed on Stryker vehicles to protect against unanticipated attacks by Iraqi insurgents using low-tech weapons works against half the grenades used to assault it. The shield, installed at a base in Kuwait, is so heavy that tire pressure must be checked three times daily. Nine tires a day are changed after failing, the report says; the Army told The Post the current figure is "11 tire and wheel assemblies daily."

"The additional weight significantly impacts the handling and performance during the rainy season," says the report, which was prepared for the Center for Army Lessons Learned in Fort Leavenworth, Kan. "Mud appeared to cause strain on the engine, the drive shaft and the differentials," none of which was designed to carry the added armor.

Commanders' displays aboard the vehicles are poorly designed and do not work; none of the 100 display units in Iraq are being used because of "design and functionality shortfalls," the report states. The vehicle's computers are too slow and overheat in desert temperatures or freeze up at critical moments, such as "when large units are moving at high speeds simultaneously" and overwhelm its sensors.

The main weapon system, a $157,000 grenade launcher, fails to hit targets when the vehicle is moving, contrary to its design, the report states. Its laser designator, zoom, sensors, stabilizer and rotating speed all need redesign; it does not work at night; and its console display is in black and white although "a typical warning is to watch for a certain color automobile," the report says. Some crews removed part of the launchers because they can swivel dangerously toward the squad leader's position.